Optical Wavelength Division Multiplexed (WDM) systems ideally require passive optical wavelength multiplexers and demultiplexers which have isolated pass-bands which are flat-topped so as to allow a measure of tolerance in the spectral positioning of the individual signals of the WDM system within these pass-bands. One method of multiplexing or demultiplexing channels in an optical WDM system relies upon the use of multilayer dielectric interference filters. Another relies upon Bragg reflection effects created in optical fibres. A third method, the method with which the present invention is particularly concerned, relies upon diffraction grating effects.
One form that such a diffraction grating can take for wavelength multiplexing/demultiplexing is the form described in EP 0 254 453, which also refers, with particular reference to its FIG. 5, to the possibility of having a tandem arrangement of two diffraction gratings arranged to provide a combined intensity transfer function that is the product of the intensity transfer function of its component diffraction grating 40 with that of its component diffraction grating 42.
An alternative form that such a diffraction grating can take is an optical waveguide grating that includes a set of optical waveguides in side-by-side array, each extending from one end of the array to the other, and being of uniformly incrementally greater optical path length from the shortest at one side of the array to the longest at the other. Such an optical grating constitutes a component of the multiplexer described by C Dragone et al., `Integrated Optics N.times.N Multiplexer on Silicon`, IEEE Photonics Technology Letters, Vol. 3, No. 10, October 1991, pages 896-9. Referring to FIG. 1, the basic components of a 4-port version of such a multiplexer comprise an optical waveguide grating, indicated generally at 10, where two ends are optically coupled by radiative stars, indicated schematically at 11 and 12, respectively with input and output sets of waveguides 13 and 14. Monochromatic light launched into one of the waveguides of set 13 spreads out in radiative star 11 to illuminate the input ends of all the waveguides of the grating 10. At the far end of the grating 10 the field components of the emergent light interfere coherently in the far-field to produce a single bright spot at the far side of the radiative star 12. Scanning the wavelength of the light causes a slip in the phase relationship of these field components, with the result that the bright spot traverses the inboard ends of the output set of waveguides 14 linearly with wavelengths as depicted at 15. If the mode size of the waveguides 14 is well matched with the size of the bright spot, then efficient coupling occurs at each of the wavelengths at which the bright spot precisely registers with one of those waveguides 14. Either side of these specific wavelengths the power falls off in a typically Gaussian manner as depicted at 15. While this may allow acceptable extinction to be achieved between channels, it is far from the ideal of a flat-topped response.
A tandem arrangement of this alternative form of diffraction grating can also be constructed, an example of such an arrangement being described in EP 0 591 042 with particular reference to its FIG. 3. This tandem arrangement similarly provides a combined intensity transfer function that is the product of the intensity transfer functions of its two component diffraction gratings. The response of this tandem arrangement also provides a typically Gaussian fall off in power that is similarly far from the ideal of a flat-topped response.
A construction of multiplexer/demultiplexer that also uses a tandem arrangement of optical waveguide gratings, but which is capable of achieving a response that is more nearly flat-topped without introducing an excessive insertion loss is described in the specification of WO 98/04944, to which specification attention is specifically directed and its teaching incorporated herein by reference.
WO 98/04944 discusses the desirability, at least in some circumstances, of incorporating some form of field stop between the two optical waveguide diffraction gratings of a tandem pair. In bulk-optics composite optical systems, a field stop is typically constituted by a diaphragm of opaque, usually black, material that is provided with an aperture through which light is able to pass. The size and shape of this aperture determines the cross-sectional area of the pencil of light that is able to couple, by penetration through the aperture, from an optical system on one side of the diaphragm into an optical system on the other side. The function of the field stop is to define the boundaries of this cross-sectional area, typically for the purpose of permitting the passage of as much light as is consistent with maintaining a desired quality of imaging. If the two optical systems are optically coupled by means of a reflector, this same function of defining the boundaries of the cross-sectional area of the pencil of light that optically couples the two systems may alternatively be effected by definition of the boundaries of the reflector itself. The sole example of field stop specifically exemplified in WO 98/04944 is a field stop of this latter (reflection type) kind.